1. Field of the Invention
The present invention is related to the use of polysaccharide derivatives in vaccines and immunostimulating compositions. The adjuvants are derivatives of polysaccharides recognized by antigen presenting cells (APCs).
2. Related Art
Adjuvants have utility in activating the immune system to increase the efficacy of preventative and therapeutic vaccines. Immunoadjuvants have applications in: (1) the non-specific stimulation of host resistance against infection and cancer, (2) the potentiation of preventative vaccine immunogenicity, and (3) the potentiation of therapeutic vaccine immunogenicity. These adjuvants may preferentially enhance cell-mediated immune responses (T cell responses, delayed hypersensitivity), humoral responses (B cell responses, antibody production), or both. Stimulation of humoral immunity is important for prevention of bacterial infections, some viral infections, as well as in therapy of circulating cancers. Cellular immunity is of major importance for solid tumor cancer therapy and some viral diseases.
After an initial stimulation by a foreign agent or antigen (such as viruses, bacteria, or parasites), the immune system usually recognizes and reacts to the agent with an accelerated response upon re-exposure. This enhanced response forms the basis for the enormous success of vaccination for disease prevention. However, the initial immune response to a foreign antigen requires several days for full response, which is insufficient for protection against infections by highly virulent organisms. A way to achieve a faster protective immune response is by vaccination or immunization with a pathogen, which is usually attenuated or dead. However, in many cases immunization with killed microorganisms or with pure antigens elicits a poor short term immune response with weak or no cell-mediated immunity produced at all. In many cases this poor immune response can be modified by the addition of adjuvants to the antigen preparation. Several polysaccharides (carbohydrate polymers) of mannose (e.g. mannans), β(1,3) glucose (e.g. glucans), β(1,4) acetylated mannose (acemannans), β(1,4) N-acetyl-glucosamine (chitins), and heteropolysaccharides, such as rhamnogalacturonans (pectins), have been shown to stimulate the immune system. Antigen presenting cells (APCs) have specific cell-surface-receptors which recognize and bind the sugar moieties of these and other polysaccharides. Antigen presenting cells (APCs), such as dendritic cells and some macrophages, are responsible for taking up antigens and processing them to small peptides in endolysosomes. Processed antigens are expressed on the surface of APCs in conjunction with class II MHC. Specifically, reactive T cells recognize antigen and class II MHC simultaneously, yielding immune responses that are class II MHC restricted. B cells are stimulated by processed antigens to produce antibodies. These APC surface-receptors (such as the macrophage mannose receptor and its homologous receptor DEC-205 from dendritic cells) are transmembrane proteins that mediate endocytosis and apparently play a role in the process of antigen presentation (Stahl, P. D., Current Opinion in Immunology 4:49 (1992); Jiang, W., et al., Nature 375:151 (1995)). Binding of these polysaccharides to such receptors apparently induces immunostimulation, as shown by the increase in phagocytosis, proliferative responses, release of cytokines, and other activities of the immune system. Because of this immunostimulatory activity, these polysaccharides have been proposed as vaccine adjuvants.
Polysaccharide adjuvants exert an immunomodulating effect by modifying cytokine production, such as upregulating IL-1, and causing a moderate Th1 response. The immune response produced by the Th1 subset of CD4+T cells induces complement fixing antibodies as well as strong, delayed-type hypersensitivity (DTH) reactions associated with γ-IFN, IL-2 and IL-12. Polysaccharides' effects on the native protein conformation are moderate, preserving the conformational epitopes necessary to elicit a neutralizing antibody response. However, because these adjuvants cannot allow exogenous antigens to be processed via the endogenous pathway, they do not induce a cytotoxic T lymphocyte (CTL) response. Because APCs have cell-surface-receptors specific for certain carbohydrate moieties, the targeting and delivery to these cells of antigens associated with these sugar moieties can be significantly enhanced. Apparently, the role of sugar moieties in the targeting of antigen delivery is not limited to polysaccharide adjuvants. For instance, the modification of quillajasaponin carbohydrate side-chains by periodic acid oxidation results in a loss of their adjuvanticity. Presumably, this results because of the loss of their targeting capacity.
Although the adjuvant properties of certain polysaccharides have been known for some time, their use has been largely limited to research applications. For instance, it has been shown that glucans can induce an anti-tumor response in mice, and have a preventive effect on acute sepsis. These effects are dependent on the glucans' molecular weight and their degree of branching. Mannans are other polysaccharides with adjuvant activity which presumably exert their effect after binding to the macrophage mannose cell-surface-receptor. Recently, it has been shown that conjugation of a protein antigen to mannan under oxidizing conditions resulted in a cell-mediated immune response (Apostolopoulos, V. et al., Vaccine 14:930 (1996)). However, protein antigens conjugated to mannans under non-oxidative conditions, i.e. without aldehyde formation, elicited only humoral immunity (Okawa, Y. et al., J. Immunol. Meth. 142:127 (1992)) and (Apostolopoulos, V. et al., Proc. Natl. Acad. Sci. USA. 92:10128 (1995)). Stimulation of T-cell immunity has also been achieved by generating with galactose oxidase under experimental conditions, aldehydes in the galactosyl residues of cell-surface polysaccharides (Zeng, B., et al., Science 256:1560 (1992)). However, this immunostimulation was not reproducible (Rhodes, J. Immunol. Today 17:436 (1996)). These results highlight the problems associated with aldehyde instability and/or the inefficient production of aldehydes by enzymatic oxidation.
It is clinically and economically important for the vaccine industry to have new and effective adjuvants. The development of novel adjuvants that target antigen presenting cells and provide co-stimulatory signals to stimulate T-cell immunity is the subject of the present patent disclosure.